Method and apparatus for observing, detecting and correcting periodic structures in a moving web

ABSTRACT

The diffraction pattern, produced by the periodic structure of a moving web is stationary. Characteristics of the periodic structure may be observed in the diffraction pattern. Deviations from a norm of a characteristic of the periodic structure are detected by imaging the diffraction pattern on a mask which is a matched filter for the characteristic. In a specific application the periodic structure is the weft of a moving cloth web, the characteristic is weft alignment, and the matched filter mask is a slit. In order to obtain an alternating current output signal, the cloth is illuminated by an oscillating slit of monochromatic light. Misalignments of the weft produce rotations in the diffraction image which may be detected by means of a pair of photodetectors located behind a slit in the plane of the diffraction image. As in the prior art a plurality of such weft misalignment detectors may be located across the moving cloth web and the angular deviations of the weft at each detector compared to produce a control signal for controlling existing automatic devices for making weft misalignment corrections. In an alternative embodiment, relative rotation between the diffraction image and a slit produces a peak output from a photodetector when the slit is aligned with a linear pattern in the diffraction image. The output of a pulse generator driven in synchronism with the slit or image is counted from an initial reference position until the peak is attained to provide a digital output of the angular orientation of the pattern. In another embodiment of the invention, the detector takes the form of a photochromic sheet illuminated by an ultraviolet diffraction pattern and collecting optics focusing the ultraviolet radiation passing through the photochromic sheet upon a photodetector. A perfect moving web will produce a stationary diffraction pattern forming an opaque mask in the photochromic sheet and substantially no ultraviolet light will fall on the photodetector. Short term deviations from normal will cause diffracted light to pass through the transparent areas of the photochromic material producing a signal at the photodetector. The detector may also be illuminated with infrared radiation which tends to bleach it. The frequency of the periodicity of the moving web may be detected by combining diffracted light with a reference beam of light from the same source of illumination or a source of illumination coherent therewith to thereby produce a beat amplitude at a photodetector in synchronism with the passage of the periodic structure.

United States Patent 72] Inventor Peter II. Langenbeck Norwalk, Conn.

[21] Appl. No. 866,587

[22] Filed Oct. 15, 1969 [45] Patented Jan. 4, 1972 73] Assignee ThePerkin-Elmer Corporation Norwalk, Conn.

[54] METHOD AND APPARATUS FOR OBSERVING,

DETECTING AND CORRECTING PERIODIC Primary Exa mineF-Archie R. BorcheltAssistant ExaminerD. C. Nelms Attorney-Edward R. Hyde, Jr.

ABSTRACT: The diffraction pattern, produced by the periodic structure ofa moving web is stationary. Characteristics of the periodic structuremay be observed in the diffraction pattern. Deviations from a norm of acharacteristic of the periodic structure are detected by imaging thediffraction pattern on a mask which is a matched filter for thecharacteristic. In a specific application the periodic structure is theweft of a moving cloth web, the characteristic is weft alignment, andthe matched filter mask is a slit. In order to obtain an alternatingcurrent output signal, the cloth is illuminated by an oscillating slitof monochromatic light. Misalignments of the weft produce rotations inthe diffraction image which may be detected by means of a pair ofphotodetectors located behind a slit in the plane of the diffractionimage. As in the prior art a plurality of such weft misalignmentdetectors may be located across the moving cloth web and the angulardeviations of the weft at each detector compared to produce a controlsignal for controlling existing automatic devices for making weftmisalignment corrections.

In an alternative embodiment, relative rotation between the diffractionimage and a slit produces a peak output from a photodetector when theslit is aligned with a linear pattern in the diffraction image. Theoutput of a pulse generator driven in synchronism with the slit or imageis counted from an initial reference position until the peak is attainedto provide a digital output of the angular orientation of the pattern.

In another embodiment of the invention, the detector takes the form of aphotochromic sheet illuminated by an ultraviolet difiraction pattern andcollecting optics focusing the ultraviolet radiation passing through thephotochromic sheet upon a photodetector. A perfect moving web willproduce a stationary diffraction pattern forming an opaque mask in thephotochromic sheet and substantially no ultraviolet light will fall onthe photodetector. Short term deviations from normal will causediffracted light to pass through the transparent areas of thephotochromic material producing a signal at thephotodetector. Thedetector may also be illuminated with in-' frared radiation which tendsto bleach it.

The frequency of the periodicity of the moving web may be detected bycombining diffracted light with a reference beam of light from the samesource of illumination or a source of illumination coherent therewith tothereby produce a beat amplitude at a photodetector in synchronism withthe passage of the periodic structure.

PATENTEUJm 4:912 slsaslos? SHEET 1 OF 4 INVENTOR PETER H.LANGENBECKDWARD R.HY0E 5 ATTORNEY msmmm 4m: 31633031 sum 2 or 4 FIG. 4

PATENTEDJAN 4m:

. SHEET 3 OF 4 CONTROL SIGNAL I48 READ OUT COUNTER PULSE GENERATORMETHOD AND APPARATUS FOR OBSERVING. DETECTING AND CORRECTING PERIODICSTRUCTURES IN A MOVING WEB SUMMARY OF THE INVENTION This inventionrelates to the observation of, detection of deviations, and correctionof deviations, in periodic structures of moving webs, particularlytextile fabrics. The invention is based upon my observation that thediffraction pattern formed by a moving web having a periodic structureis stationary if the periodic structure is perfect. Thus deviations insaid periodic structure from a norm may be observed as temporal changesin the diffraction pattern. The deviation may be removed and a controlsignal generated for controlling apparatus to correct such deviations.

During weaving and processing, textile fabrics become distorted anddeformed. Usually the warp threads stay aligned parallel to the line ofmotion in the processing machinery. However, the weft or cross threadsoften become skewed and bowed. Many prior art devices have been proposedfor automatically detecting and correcting weft misalignment. However,this is not an easy matter especially when one realizes that the fabricis typically moving at a rate of 100 yards per minute through theprocessing machinery and the apparatus is subjected to the deleteriouseffects of lint and various corrosive and caustic processing chemicals.

Commercial prior art weft detectors are based upon differential threadcounting. In these instruments the weft threads are counted at aplurality of stations located across the web. Usually a slit is locatedclose to the moving fabric and each time the weft thread crosses theslit a pulse is generated. The frequencies of the pulses from theplurality of stations are compared in a complex electronic system and anoutput signal is produced which is either displayed to an operator whothen manually controls a device which makes bow and selvedge correctionsjust prior to rolling up the fabric or the signal is used toautomatically control a similar device.

A large variety of fabrics, such as twill, have a diagonal periodicstructure which causes erroneous signals. Certain printed patterns alsohave this effect. The different speeds at which various fabrics areprocessed result in different frequency outputs from the prior artdetectors adversely affecting their response and increasing the cost andcomplexity of the comparing circuitry. The illuminating slits of theseprior art detectors must be opened or closed to match the diameter ofthe weft in the fabric being processed. The slits are located close tothe cloth and often become clogged with lint. When the path of the clothmoves toward and away from the slits, as often occurs, erroneousreadings are produced. Thus, prior art weft misalignment detectors arecumbersome and unreliable. Furthermore, knitted goods which have in thepast few years become an increasingly large portion of the total textileoutput cannot be processed in existing apparatus.

Thus, there is a need for a weft alignment detector which is essentiallycontactless, not adversely affected by the textile weaving andprocessing environment, and which does not require calibration forprocessing widely varying types of fabrics.

Similarly there is a need for such apparatus for observing, detectingand correcting periodic structures in other moving webs, such asnonwoven feltlike fabrics, paper, plastic, sheet metal and the like.

It is therefore a principal object of the invention to provide a methodand apparatus for observing periodic structures in a moving web.

Another object of the invention is to provide a method and apparatus ofthe above character for detecting deviations from a norm in the periodicstructure.

Still another object of the invention is to provide a method andapparatus of the above character for correcting such deviations.

Yet another object of the invention is to provide a method and apparatusof the above character requiring no contact with the moving web.

A further object of the invention is to provide a method and apparatusof the above character which is not adversely affected by theenvironment of the moving web being observed.

Another object of the invention is to provide a method and apparatus ofthe above character producing signals which are capable of simpleprocessing and interpretation.

Still another object of the invention is to provide a method andapparatus of the above character in which a single isolatedcharacteristic of the periodic structure of the moving web may beobserved irrespective of widely varying other characteristics of themoving web.

A further object of the invention is to provide a method and apparatusof the above character in which the frequency or wavelength of theperiodic structure may also be determined.

A still further object of the invention is to provide a method andapparatus of the above character for detecting and cor recting weftmisalignments and for counting weft threads in a moving cloth web.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises several steps and the relation ofone or more of such steps with respect to each of the others, andapparatus embodying features of construction, combinations of elementsand arrangement of parts which are adapted to effect such steps, all asexemplified in the following detailed disclosure. The scope of theinvention is indicated in the claims.

THE DRAWINGS For a fuller understanding of the nature and objects of theinvention, reference should be had to the following detailed descriptiontaken in connection with the accompanying drawings, in which:

FIG. 1 is a side view, partially in diagrammatic form, of fabricmisalignment detecting and correcting apparatus according to theinvention;

FIGS. 2A, 2B and 2C are diagrammatic views of cloth having weftdistortion which may be corrected by means of the method and apparatusof the invention;

FIG. 3 is a diagram of an optical system illustrating the principals ofthe invention;

FIG. 4 is a representation of the diffraction pattern produced by theoptical system of FIG. 3;

FIG. 5 is a diagram of the optical system of the apparatus of FIG. 1;

FIG. 6 is a detailed perspective view, partially cutaway, of a portionof the apparatus of FIG. ll;

FIG. 7 is a detailed cross-sectional view of a portion of the apparatusof FIG. ll taken along the line 7-7 of FIG. 6;

FIG. 8 is a diagram illustrating a principal of operation of theapparatus of FIGS. 5 through 7;

FIG. 9 is an optical and electrical schematic diagram of a detector foruse in the optical system illustrated in FIG. 3 for producing a digitaloutput; and,

FIG. 10 is a diagram of an alternative form of detector for use in theoptical system of FIG. 3.

The same reference characters refer to the same elements throughout theseveral views of the drawings.

SPECIFIC DESCRIPTION Now referring to FIG. 1 wherein apparatus accordingto the invention is shown in generally diagrammatic form, a movingfabric web 12 is unrolled from a roll 14 and passes through processingapparatus 16. The web 12 is: then fed to an existing mechanical device18 for making bow and selvedge corrections. Such devices 18 may bemanually controlled or automatically controlled.

Before passing through apparatus 18 the fabric passes through apparatusaccording to the invention, generally indicated at 20, and is rolled uponto roll 22. The apparatus 20 according to the invention is mounted torigid support structure 24 to one side of the fabric web 12. A source ofradiation 26 is located adjacent to the web 12 and the radiation passestherethrough to detector and control signal producing apparatus 28. Thedetector and control signal producing apparatus '28 may produce anelectrical signal on line 30 supplied to readout apparatus 32 forobservation by a human operator 34. The human operator may then controlthe correcting apparatus 18 by means of control box 36. Alternativelythe detecting and control apparatus may produce automatic controlsignals on line 38 which are supplied directly to automatic correctingapparatus 18.

In FIGS. 2A, 2B and 2C there is illustrated typical weft misalignments.In FIG. 2A the weft threads 40 are skewed with respect to the warpthreads 42. In FIG. 2B the weft threads 40 are bowed with respect to thewarp threads 42. And, in FIG. 2C S" distortion of the weft 40 withrespect to the warp 42 is shown.

Now referring to FIG. 3 wherein a generalized optical system of themethod and apparatus of my invention is shown,

a light source 44 is located behind a color filter 46 and a ground glassdiffusing screen 48. Substantially monochromatic light is thereforeallowed to fall on a slit or pinhole in diaphragm 50. This light iscollimated by collimating lens 52 and illuminates the moving web 54.Light, generally indicated at 56 by dotted lines having been diffractedby passing through the moving web is focused by means of lens 58 to formthe Fraunhofer diffraction image of the moving web 54 at the diffractionimage plane 60. The image is enlarged by lens 62 and reimaged at theplane of detection 64. As will be understood by those skilled in theart, when the opening in diaphragm 50 is a slit the diffraction patternformed at image planes 60 and 64 is modified. The purpose of thismodification will become apparent from the discussion below.

As is also understood by those skilled in the art, all that is necessaryto form the diffraction pattern at planes 60 and 64 is illumination ofthe web 54 with spatially coherent radiation. Substantiallymonochromatic illumination which is fairly well collimated has thisproperty and may be achieved by use of the collimating lens 52 or bylocating the light source at a large distance from the moving web 54.The spatially coherent illumination may under certain circumstances beadvantageously provided by the use ofa laser.

The diffraction pattern observed at diffraction image planes 60 and 64is diagrammatically shown in FIG. 4. The diffraction pattern 66 shown inFIG. 4 comprises a central maximum 68 of undiffracted light,horizontally aligned first order maxima 70 and 72 of diffracted lightspaced apart in accordance with the warp thread density measured inthreads per unit length. The warp is aligned vertically and the wefthorizontally as in FIGS. 2A, 2B and 2C. Vertically aligned first ordermaxima 74 and 76 are spaced apart proportional to the weft threaddensity. Higher order maxima are also shown. To produce the pattern 66of FIG. 4, the opening in diaphragm 50 (FIG. 3) is a slit aligned withthe weft. Its effect is to smear the maxima in a direction aligned withthe slit and the weft. My invention depends upon the fact that theamplitude and thus the brightness of the diffraction image of FIG. 4 isunaffected by the speed of the fabric web 54 shown in FIG. 3.

It will be understood by those skilled in the art that othercharacteristics of the cloth web 54 may be determined from thediffraction pattern 66, such as thread thickness, which is proportionalto the intensity ratios of the subsequent diffraction orders and fabricstructures, such as sateen and twill, which will produce characteristicdistortions in the normal diffraction pattern shown in FIG. 4.Furthermore, alignment of the fabric may be determined, in that thediffraction pattern 66 will rotate to the same angle as the fabric 54 isrotated. When the weft is misaligned, the diffraction pattern 66 will beskewed through the same angle.

An optical system generally indicated at 69 in FIG. 5 is capable ofdetecting this angular misalignment of the weft. The moving fabric web54 passes through the system as shown. Light source 44 illuminatesfilter 46 and ground glass diffuser 48. The light therefrom isconcentrated by lens 71 on slit 73. The rotating multifaceted mirror 75causes light from the slit 73 to be repeatedly swept across lens 77which focuses an oscillating image of slit 73 at sweep limiting slit 78.The oscillation is needed only for obtaining an electric alternatingcurrent signal, not for visual display. The light passing through slit78 is reflected at mirror 80, through the fabric 54 and reflected bymirror 82 to the diffraction image forming lens 58. The diffractionpattern produced at plane 60 is enlarged by lens 62 and focused on aslit 84 parallel to slit 73 and shown in perspective in FIG. 5.Photocell 86 is located on the optical axis 88 to receive the light fromthe central maximum of undiffracted light 68 shown in FIG. 4 andphotocell 90 is located to receive light from one of the first ordermaxima 70 or 72 produced by the average fabric processed by the machine.

As previously stated, for detecting weft alignment one is interested inorientation of the diffraction pattern only. In order to measure withthe same photo-optic detectors, fabric of different warp densities, one,therefore, best produces the pattern in such a way that the individualdiffraction orders appear oblong enough to overlap one another (as inFIG. 4) rather than circular. This can be done either by using a slit 73for illumination or by using a cylinder lens (for example, lens 62). Thedimensions of the cylinder lens are such that, for the occurring threaddensities, the oblong diffraction orders always overlap as in FIG. 4.

Similarly, rather than oscillating the slit 73, the pattern could beoscillated by oscillating the cylindrical lens or the slit 84 anddetectors 86 and 90 could be oscillated with respect to a stationaryimage of the diffraction pattern.

Those skilled in the art will also understand that in order to produce abright diffraction pattern the diffracted light collecting lens 58should be as close to the fabric 54 as possible. However, this distanceis not critical, and the larger the diameter of lens 58, the fartheraway it may be.

Those skilled in the art will also realize that the slit 78 limits thescanning such that the diffraction pattern 66 imaged at the slit 84oscillates up and down less than the distance between the first ordervertical maxima 74 and 76. Thus, the crossing of the central maximum 68across the slit 84 will produce a reference pulse in photodetector 86and the time of passage of the first order maximum 70 across the slit 84detected by the photodetector 90 will be an indication of the amount ofskew in the weft. If there is no misalignment, the two signals will beproduced simultaneously. The time difference between the two signalswill be proportional to the skew angle.

The apparatus 20 shown in FIG. I utilizes the optical system shown inFIG. 5 and is shown in greater detail in FIGS. 6 and 7. Referring toFIGS. 5 and 6, housing 92 is rigidly mounted to support 24 (shown inFIG. 1). A plurality of alignment detector illuminators are locatedacross the width of the fabric as generally indicated at 94, 96 and 98.Each illuminator comprises lamp 44, filter 46 (which may be a Wratten),ground glass 48, lens 70 and slit 12 mounted in housing 99. The motor100 drives a plurality of multifaceted mirrors 74. Lens 76 and slit 78at each station are mounted in a housing 102 and the whole is protectedfrom the intrusion of dust by means of glass plates 104. It will benoted that the mirrors 80 and 82 are not used in the system of FIGS. 6and 7.

As shown in detail in FIG. 7, the collecting optics of each stationrather than being a simple lens 58 may be a Cassegrainian system mountedin a housing 106. This system comprises protective glass plate 108,collecting mirror 110, reflecting mirror 112 forming the diffractionpattern at image plane 60, magnifying lens 62, slit 84 and detectors 86and 90.

The relationship between weft alignment and the signals produced by thephotodetectors 86 and 90 is illustrated in FIG. 8. On the left the slit84 is shown. The diffraction pattern oscillates up and down as shown at113 with respect to the slit 84. This may be at a frequency of 35 cyclesper second, for example.

The patterns for perfect weft alignment 114, a small amount of clockwiserotation 116, a small amount of counterclockwise rotation 118 and alarge amount of counterclockwise rotation 120 are shown. The positionsof the photodetectors are indicated by circles 90 and 86 and the maximaof the diffraction pattern 66 illustrated in FIG. 4 are indicated by Xs68, 70 and 72.

The electrical signals provided by the photocells 86 and 90 at eachalignment condition are illustrated on the right of FIG. 8. In perfectalignment the two signals occur simultaneously and if they are addedtogether as shown in F [6.8, a maxima 122 is produced each time themaxima 68 and 70 cross the slit 84. A small amount of clockwisemisalignment will cause the first order maximum 70 to cross the slit andphotocell 90 before the zero order maximum 68 crosses the photocell 86.Thus a small intensity peak 124 from the photocell 90 will precede alarger intensity peak 126 from the photocell 86 each time the patterncrosses the slit 84.

Similarly for counterclockwise misalignment the zero order maximum 68will cross photocell 86 before the first order maximum 70 crosses thephotocell 90. Thus, the large amplitude pulse 126 will precede the smallamplitude pulse 124. Larger misalignments will cause a greater timedifference between receipt of the two pulses 124 and 126. Thus, as shownat 120, for a large counterclockwise misalignment, maximum 126 willoccur quite a long time before maximum 124.

As will be understood by those skilled in the art, the signalsillustrated in FIG. 8 may be displayed on an oscilloscope either on thesame line or on different lines on a dual-trace oscilloscope. A scale,such as generally indicated at 128, may be provided of, for example,degrees of misalignment, as shown. Thus, referring to FIG. 1, thedisplay 132 may be such an oscilloscope.

Furthermore, the time difference between the pulses 124 and 126 from theseveral stations 94, 96, 98 may be compared and converted to appropriatecontrol signals supplied to automatic correcting apparatus 18 on lines38 as shown in FIG. 1.

It will be understood that the apparatus illustrated in FIGS. -8produces an analog output signal. That is, the signal is an analogfunction (time) proportional to the angular misalignment of the weft. Inmany cases it is desirable to provide a digital signal. Apparatus fordoing this is illustrated in FIG. 9.

In FIG. 9 apparatus for observing the diffraction pattern produced asshown in FIG. 3 is generally indicated at 130. As shown in FIG. 9, aslit 132 at the diffraction image plane 64 is rotated by means of amotor 134 through a gear 136. Light passing through the rotating slit132 is collected by lens 138 and falls on photodetector 140. When slit132 is aligned with the zero order maximum 68 and the first order maxima70 and 72, a maximum amount of light will fall on the photodetector 140.The orientation of the slit 132 at this time is then the same as theweft. This angle may be determined by means ofa pulse generator orangular encoder 142 producing pulses corresponding to fixed intervalangular rotations of the slit 132. The pulses are counted by controlcircuitry 144 from the time the slit 132 is at a zero angular positionuntil the maximum signal is produced by photodetector 140. Thus, adigital readout 146 may be provided or a digital signal on line 148.

Another form of deviation detector is generally indicated at 150 in FIG.10. It comprises a photochromic sheet 152 located at the diffractionplane 60 or 64 in FIG. 3. Photochromic sheet 152 darkens when exposed tothe illumination from light source 44. The light forming the diffractionimage will cause the photochromic detector 152 to become opaque in apattern exactly corresponding thereto. Thus, a perfectly periodic webwill produce an invarying diffraction pattern and after a certain amountof time depending on the response time of the photochromic material 152,no light will be collected by collecting lens 154 and no light will fallon photodetector 156. If a defect occurs in the periodic structure, thediffraction pattern will shift and some light will pass throughphotochromic sheet 152. This light will be detected by photodetector 156indicating the presence of a defect. As most photochromic materials aredarkened by ultraviolet light, light source 44 supplies suchillumination in this embodiment of the invention. The darkening of thephotochromic detector caused by the deviated diffraction pattern may bebleached out by illuminating the same with light from an infrared source158 as infrared light bleaches the photochromic material 152. This willquickly bleach darkenings caused by deviations occurring for a shorttime. The infrared and ultraviolet sources may be interchanged in whichcase deviations would decrease the amount of radiation reaching thedetector 156 rather than increasing it.

Again referring to FIG. 3, the periodicity of the structure passingthrough the apparatus of the invention may be detected by combiningreference interfering light with light forming a diffraction maximum. Aswill be understood by those skilled in the art, this is because: thephase of the light forming each diffraction maximum changes insynchronism with the passage of the periodic structure while theamplitude thereof remains unchanged. This phase may be detected byinterference with a beam oflight having a fixed reference phase. One wayto produce this is to use the half-silvered mirrors 158 and 160 shown inFIG. 3. Mirror 158 produces a reference beam which may be guided bymirrors 162 and 164, focused by lens 166, and directed by half'silveredmirror 160 onto one of the diffraction maxima at plane 64. The lightamplitude variations resulting from the interference of the two beamsmay then be detected by photodetector 90. In this manner, for example,the speed of the moving fabric 54 may be determined from the number ofweft threads counted.

It should be understood that the reference beam of FIG. 3 need not beextracted directly from the illuminating light source if, for example,two synchronized sources, such as lasers, are used, one as theilluminating source and one as the source of the reference beam.

It will thus be seen that one aspect of my invention is the ob servationof the characteristics of a moving web by means of observing thediffraction pattern formed by the moving web. Although the pattern isformed in the apparatus hereinbefore disclosed by passing the radiationthrough the moving web, the diffraction pattern may also be formed byreflecting the radiation from the moving web without departing from theprincipals of my invention. One advantage of my method is that theinformation detected depends upon the average characteristics of the webover the entire relatively large area illuminated. Thus, short termvariations may be ignored and large movements of the web towards andaway from the illuminating source or the detector are not significant.Furthermore, no changes in the illuminator or calibrations of thedetector need be made even for large variations in the characteristicsof the moving web.

While the specific moving web disclosed herein is textile fabric, theinvention is also applicable to other moving webs having periodicstructures. Similarly, while for the sake of simplicity I have discussedwoven fabrics, the invention is particularly applicable to knittedfabrics. Warp and weft as used herein therefore also refer to thecorresponding orthogonally aligned threads or structures of knittedfabrics.

While the illuminating radiation disclosed herein is light, it will beunderstood that other radiation, such as Xaays, ultrasonic rays,microwaves, or the like, may also be used together with appropriatemonochromatizing, diffracting, focusing and detecting elements.

Although in one embodiment of the invention I have disclosed phototropicmaterial, it should be understood that other forms of radiotropicmaterial might be used in combination with any appropriate detector ofthe radiation as affected by the radiotropic material.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in carrying out the above method andin the constructions set forth without departing from the scope of theinvention, it is intended that all matter contained in the abovedescription or shown in the accompanying drawings shall be interpretedas illustrative and not in a limiting sense.

Having described my invention, what I claim as new and desire to secureby Letters Patent is:

l. The method of detecting deviations in a moving web from a normcomprising the steps of:

A. illuminating the moving web with spatially coherent radiation;

B. forming a diffraction image of the web in radiotropic material withthe illuminating radiation which has interacted with said web; and,

C. detecting temporal changes in radiation which has interacted withsaid material.

2. The method defined in claim 1 wherein said illuminating radiationconsists of a first frequency band and said radiotropic materialproduces opposite radiotropic responses to radiation of said firstfrequency band and radiation of a second frequency band differing fromsaid first frequency band; and the additional step of:

D. illuminating said radiotropic material with radiation of said secondfrequency band.

3. The method defined in claim 2 wherein said material is phototropic,said illuminating radiation is infrared light, and said second frequencyband is ultraviolet light.

4. The method defined in claim 2 wherein said material is phototropic,said illuminating radiation is ultraviolet light, and said secondfrequency band is infrared light.

5. The method of detecting deviations in a moving web from a normcomprising the steps of:

A. illuminating the moving web with spatially coherent radiation;

B. forming a diffraction image of the web with the illuminatingradiation which has interacted with said web;

C. additionally illuminating said diffraction image with radiationcomprising a reference phase component; and,

D. detecting the temporal interference pattern formed by said additionalradiation and said diffraction image.

6. Apparatus for observing a characteristic of a periodic structure of amoving web comprising:

A. a source of spatially coherent illumination for illuminating astationary area through which the web passes;

B. an optical system for forming a diffraction image of said web;

C. means for combining at least one diffraction order of saiddiffraction image with reference illumination coherent therewith; and,

D. a detector responsive to the time amplitude interference patternproduced thereby.

7. Apparatus for observing a characteristic of a periodic structure of amoving web comprising:

A. a source of spatially coherent illumination for illuminating an areathrough which the web passes;

B. an optical system for forming with said illumination a diffractionimage of said web;

C. a detector for detecting rotation of said diffraction imagecomprising:

a. a slit at said diffraction image rotating about an axis passingthrough the zero diffraction order,

b. a photodetector on said axis for receiving the light forming thediffraction image passing through said slit and for producing a signalin response thereto,

c. reference signal producing means synchronized with said rotatingslit, and

d. means for producing a signal proportional to the phase differencebetween said reference signal and said photodetector signal.

8. Apparatus as defined in claim 7 wherein said phase difference signalproducing means comprises a counter which starts counting upon receiptof said reference signal and stops counting upon the attainment of apredetermined amplitude in said photodetector signal.

9. Apparatus as defined in claim 7 wherein said source of illuminationcomprises a slit.

10. Apparatus as defined in claim 7 wherein said source of illuminationcomprises a cylindrical lens.

11. Apparatus for observing a characteristic of a periodic structure ofa moving web comprising:

A. a source of spatially coherent illumination for illuminating an areathrough which the web passes; B. an optical system for forming with saidillumination a diffraction image of said web;

C. a slit on which said image is formed; and,

D. a detector for detecting movement of said diffraction image withrespect to said slit comprising a first photodetector located to receivethe undiffracted light passing through said slit and for producing asignal in response thereto, and a second photodetector located toreceive diffracted light passing through said slit to one side of theundiffracted light and for producing a signal in response thereto, andmeans for oscillating said image with respect to said slit.

1. The method of detecting deviations in a moving web from a normcomprising the steps of: A. illuminating the moving web with spatiallycoherent radiation; B. forming a diffraction image of the web inradiotropic material with the illuminating radiation which hasinteracted with said web; and, C. detecting temporal changes inradiation which has interacted with said material.
 2. The method definedin claim 1 wherein said illuminating radiation consists of a firstfrequency band and said radiotropic material produces oppositeradiotropic responses to radiation of said first frequency band andradiation of a second frequency band differing from said first frequencyband; and the additional step of: D. illuminating said radiotropicmaterial with radiation of said second frequency band.
 3. The methoddefined in claim 2 wherein said material is phototropic, saidilluminating radiation is infrared light, and said second frequency bandis ultraviolet light.
 4. The method defined in claim 2 wherein saidmaterial is phototropic, said illuminating radiation is ultravioletlight, and said second frequency band is infrared light.
 5. The methodof detecting deviations in a moving web from a norm comprising the stepsof: A. illuminating the moving web with spatially coherent radiation; B.forming a diffraction image of the web with the illuminating radiationwhich has interacted with said web; C. additionally illuminating saiddiffraction image with radiation comprising a reference phase component;and, D. detecting the temporal interference pattern formed by saidadditional radiation and said diffraction image.
 6. Apparatus forobserving a characteristic of a periodic structure of a moving webcomprising: A. a source of spatially coherent illumination forilluminating a stationary area through which the web passes; B. anoptical system for forming a diffraction image of said web; C. means forcombining at least one diffraction order of said diffraction image withreference illumination coherent therewith; and, D. a detector responsiveto the time amplitude interference pattern produced thereby. 7.Apparatus for observing a characteristic of a periodic structure of amoving web comprising: A. a source of spatially coherent illuminationfor illuminating an area through which the web passes; B. an opticalsystem for forming with said illumination a diffraction image of saidweb; C. a detector for detecting rotation of said diffraction imagecomprising: a. a slit at said diffraction image rotating about an axispassing through the zero diffraction order, b. a photodetector on saidaxis for receiving the light forming the diffraction image passingthrough said slit and for producing a signal in response thereto, c.reference signal producing means synchronized with said rotating slit,and d. means for producing a signal proportional to the phase differencebetween said reference signal and said photodetector signal. 8.Apparatus as defined in claim 7 wherein said phase difference signalproducing means comprises a counter which starts counting upon receiptof said reference signal and stops counting upon the attainment of apredetermined amplitude in said photodetector signal.
 9. Apparatus asdefined in claim 7 wherein said source of illumination comprises a slit.10. Apparatus as defined in claim 7 wherein said source of illuminationcomprises a cylindrical lens.
 11. Apparatus for observing acHaracteristic of a periodic structure of a moving web comprising: A. asource of spatially coherent illumination for illuminating an areathrough which the web passes; B. an optical system for forming with saidillumination a diffraction image of said web; C. a slit on which saidimage is formed; and, D. a detector for detecting movement of saiddiffraction image with respect to said slit comprising a firstphotodetector located to receive the undiffracted light passing throughsaid slit and for producing a signal in response thereto, and a secondphotodetector located to receive diffracted light passing through saidslit to one side of the undiffracted light and for producing a signal inresponse thereto, and means for oscillating said image with respect tosaid slit.